London's Worst Kept Secret.

The Post Office Telecom BT Tower stands 177 metres tall (189 including aerials), & between 1962 & 1980 was the tallest building in London.
It was opened to the public by Tony Benn & Billy Butlin in 1966. Butlins ran the rotating "Top of the Tower Restaurant".
In 1971 the IRA exploded a bomb in the restaurant's toilets, but whether this was a comment on the standard of catering remains unclear.
The best item of trivia that I've learned about the tower is that it was officially a secret until 19th Feb 1993, when its existance was "revealed" in Parliament by Kate Hoey MP.
(This last fact seems so extraordinary that even someone as credulous as myself is struggling a bit with it!)